Heat-sensitive copy-sheet



ilite states Fatent t fillAZB atented Nov. 1%, i963 3,111,423HEAT-SENSITIVE COPY-SHEET Dean A. Ostlie, White Bear Lake, Minn,assignor to lviinnesota Mining and Manufacturing Company, St. Patti,Minn, a corporation of Delaware No Drawing. Filed May 16, 196i Ser. No.29,158 13 Claims. (Cl. 117-368) This invention relates to heat-sensitivecopy-sheets useful in the thermographic reproduction of difierentiallyradiation-absorptive printed or other graphic originals.

Rubeanic acid, also known as dithiooxamide, has previously beensuggested as a reactant component of chemically reactive heat-sensitivecopy-sheets, for example in conjunction with salts or soaps of metalssuch as cobalt, copper and nickel. The resulting copy-sheet provides anetfective blue image on a white background when subjected to aheat-pattern as in the thermognaphic copying process. It has been found,however, that the colorforming reaction proceeds slowly at normal roomand storage temperatures, and particularly under conditions of highhumidity, so that the sheet eventually becomes no longer visiblyheat-reactive or, it previously heatpm'nted, the image becomes illegibledue to increasing background color.

The present invention provides a highly stable but fully heat-sensitivecopy-sheet by substituting for the rubeanic acid previously employed apolymeric N,N-disubstituted rubeanic acid derivative.

A preferred reactant material is the polymeric reaction product of hexamethylene diamine and rubeanic acid in equimolar proportions,particularly when employed in conjunction with a Water insoluble metalsalt reactant such as nickel benzoate. The two reactants are intimatelydispersed in a solution of a film forming resinous or polymeric binderand the mixture is formed into a heatsensitive layer either as aself-sustaining film or as a coating on a paper-like carrier web; or thereactants may be incorporated within a fibrous web in the substantialabsence of the polymeric binder. Any of the usual polymeric binders maybe employed, particularly eitective results having been obtained with acopolymer of isobutylene and styrene (Parapol S50) and with a copolymerof polyvinyl pyrollidone and vinyl acetate.

A typical heat-sensitive copy-sheet of the invention may be representedin cross-section as follows:

Thin paper-like backing The heat-sensitive layer may be a composite oftwo separately applied adjacent layers, the rubeanic [acid derivativeand the metal salt or equivalent being separately dispersed inappropriate binder solution and separately applied to a suitablecarrier. tability is still further improved, but heat-sensitivity issomewhat lessened in such sheet structures. In another variation, afurther protective surface coating is applied over the visiblyheat-reactive layer, in which case either the carrier web or theprotective surface coating must be transparent. A preferred structureemploys a pigmented surface coating and a transparent fibrous papersheet or non-fibrous polymeric film carrier, the pigmented coatingproviding improved contrast for the colored image-forming areas of thecopy.

The addition of rubeanic acid to an organic diamine in equimolarproportions under reaction conditions produces a long chain polymericproduct by a condensation reaction involving the evolution of ammonia.In the copy-sheet and under the influence of elevated temperature, thepolymer reacts rapidly with the heavy metal ion, the reaction resultingin a visible change in the copysheet. However the reaction iseiiectively inhibited under normal storage and handling conditions, sothat the copysheet remains highly stable under such conditions.

Analogous results are obtained with other polymeric derivatives ofrubeanic acid in which adjacent rubeanic acid residues are joinedtogether at secondary nitrogen atoms through intervening skeletal chainsof carbon atoms or of carbon and oxygen or nitrogen atoms. Condensationof rubeanic acid with hexamethylene diamine results in a polymericproduct which may be represented by the structural formula 1 in which Rrepresents a terminal radical and n is an integer. in this formula theskeletal chain connecting the rubeanic acid residues consistsexclusively of carbon atoms.

An apparent exception is the condensation product of rubeanic acid withcompounds having vicinal reactive amino groups, in which reaction thereis formed a 5-membered ring structure which is non-reactive with nickelsalts or the like. Vicinal amino groups are therefore to be excluded incalculating the number of amino groups in the aliphatic primarypolyamines employed in the practice of this invention. Since anyrubeanic acid reacting with these vicinal amino groups is renderedunavailable for the intended condensation, this quantity is likewise tobe excluded in calculating the amount of reactant required.

Condensation of rubeanic acid with an amino acid such as 6-aminohexanoicacid, H N(CH CO H, followed by esterification with a glycol such asethylene glycol, produces a structure which in the specific instanceindicated may be represented as H H O O and wherein the skeletal chainconnecting the secondary nitrogen atoms of the rubeanic acid residuescontains both carbon and oxygen atoms. The same is true of the polymericderivative formed by condensation of rubeanic acid with an amino alcoholsuch as Z-aminoethanol, H N' :H cn on, followed by esterification with adicarboxylic acid such as succinic or terephthalic acid. The structureproduced with succinic acid may be represented as H H O O H Similarly,the structure of the terephthalic acid ester may be represented as Thedihydroxy intermediate may also be condensed with diisocyanates.

Compounds having more than two functional groups, for example aliphaticprimary triamines such as 1,3,5-triaminopentane, trihydroxy alcoholssuch as glycerol, and tricarboxylic acids such as trimesic acid(1,3,5-benzenetricarboxylic acid) are similarly reactive to formpolymeric derivatives of rubeanic acid or dicarboxy or dihydroxyintermediate derivatives thereof which are useful in the practice of theinvention. It will be appreciated that the compounds listed areexemplary and that other polyamines, amino alcohols, amino acids,polycarboxy acids and polyalcohols having specifically differentskeletal structures may equally well be substituted in place of thosehere named.

Nickel benzoate is a preferred example of a heavy metal salt which withthe rubeanate polymers forms normally stable but rapidly visiblyheat-sensitive copy-sheets. The salt is particularly effective inresisting discoloration under high humidity test conditions. When usedin conjunction wdth the condensation product of rubeanic acid andhexamethylene diarnine, nickel benzoate provides a substantiallycolorless copy-sheet which rapidly converts to a reddish color onheating to thermographic copying temperatures. Other heavy metal saltsmay be used with the sail e or other rubeanate polymers to provide otherimage colors, the water-insoluble salts being preferred. The followingtable indicates representative colors obtained with various metal saltsin coniunction with the polymer of rubeanic acid and hexametbylenediamine.

The following examples illustrate preferred materials and procedure inthe practice of this invention, but the invention is not limitedthereto, the scope of the invention being defined in the appendedclaims.

Example 1 A mixture of 11.6 parts by weight of hexamethylene diamine and12.8 parts of rubeanic acid is moderately heated with gentle stirringfor two hours and then permitted to remain overnight at roomtemperature. During the preliminary heating, ammonia is evolved, rapidlyat first but at a decreasing rate during the two-hour heating period.The light yellow product is washed once with water and three times withwarm ethyl alcohol and is air dried. The product melts at 8590 C.

Five parts of the polymer thus formed is intimately dispersed in asolution of 1% parts of Parapol S50 isobutylene styrene copolymer in 85parts of commercial heptane by prolonged 'rnilling in a ball mill.Separately, 20 parts of nickel benzoate is dispersed in 80 parts ofheptane by ball milling. Equal parts of the two mixtures are thenintimately mixed together and the resulting com.- position is coated ona thin paper carrier (Ml-pound snap overlay tracing paper) by means of acoating bar set at an orifice of 2 mils. The coated sheet is dried atroom temperature and provides a substantially colorless heatsensitivecopy-paper which when locally heated, as in the thermographic copyingprocess or by momentary contact with ametal test bar heated to 126 C.,provides a visibly distinct image area, in this case having a reddishcolor.

The copy-sheet prepared as above described remains visibly unchangedwhen heated for two hours at 60 C. under normal low-humidity conditions,and shows only a faint trace of color when heated for two hours at 53 C.and 90% relative humidity. In contrast, a copy-sheet GramsN,N-bis-2-hydroxyethyl dithiooxamide 5.2 TBO dianhydride 9.5

The mixture is allowed to stand at room temperature for 36 hours. It isthen poured into 1,000 ml. of ice water. A light-colored precipitateforms which is recovered on a filter, washed with water and withethanol, and dried. The light cream-colored powder melts at 163-170 C.,is insoluble in heptane, soluble in acetone and in dioxane, veryslightly soluble in ethanol. It reacts with nickel benzoate at Mil- C.to give a redbrown color. Heat-sensitive copy-sheets made as in Example1 but with the above polymer substituted for the rubeanicacid-hexamethylene diarnine polymeric reaction product of that exampleare fully stable under normal storage and handling, and provideeffective reproductions of graphic originals by thermographic copyingprocedures.

N,N-bis-2-hydroxyethyl dithiooxamide is prepared by condensation ofrubeanic acid and Z-aminoethanol, 'with elimination of ammonia.

TBO dianhydride is more specifically identified as the compound 1,4,7,S-tetrachlorobicyclo- [2,2,2] -7-octene-2,3, 5,6-tetracarboxy-licdianhydride, having the structure The polymeric product thereforepresumably has the structure As in the polymer of Example 1, thesecondary nitrogen atoms of the rubeanic acid residues are joineddirectly to termnial carbon atoms of the skeletal chains intermediatethe rubeanic acid residues.

What is claimed is as follows:

1. A heat-sensitive copy-sheet useful in the thermographic reproductionof differentially radiation-absorptive graphic originals, including avisibly heat-sensitive layer comprising a salt of a heavy metal and along chain polymeric N,N-disubstituted rubeanic acid derivative.

2. A heat-sensitive copy-sheet useful in the thermographic reproductionof dilferentially radiation-absorptive graphic originals, including avisibly heat-sensitive layer comprising a salt of a heavy metal and along chain polymeric N,N'-disubstituted rubeanic acid condensationderivative of an aliphatic primary diamine.

3. A heat-sensitive copy-sheet useful in the thermographic reproductionof dilterentially radiation-absorptive graphic originals, including avisibly heat-sensitive layer comprising a salt of a heavy metal and along chain polymeric N,N-disubstituted nibeanic acid condensationderivative wherein the rubeanic acid residues are joined at thesecondary nitrogen atoms by a skeletal chain containing carbon andoxygen atoms.

4. A heat-sensitive copy-sheet useful in the thermographic reproductionof difierentially radiation-absorptive graphic originals, including avisibly heat-sensitive layer comprising a nickel salt and a long chainpolymeric N,N'- disubstituted rubeanic acid derivative.

5. A heat-sensitive copy-sheet useful in the thermographic reproductionof difierentially radiation-absorptive graphic originals, including avisibly heat-sensitive layer comprising nickel benzoate and a long chainpolymeric N,N-disubstituted condensation derivative of rubeanic acid andhexamethylene diamine.

6. A heat-sensitive copy-sheet useful in the thermographic reproductionof differentially radiation-absorptive graphic originals, including avisibly heat-sensitive layer comprising nickel benzoate and a long chainpolymeric N,N'-disubstituted rubeanic acid polyester condensationderivative wherein the rubeanic acid residues are joined at thesecondary nitrogen atoms by a skeletal chain containing carbon andoxygen atoms.

7. A heat-sensitive copy-sheet useful in the thermographic reproductionof differentially radiation-absorptive graphic originals, including avisibly heat-sensitive layer comprising nickel benzoate and a long chainpolymeric N,N-disubstituted polyester condensation derivative ofN,N'-bis-2-hydroxyethyl dithiooxamide and a tetracarboxylic dianhydride.

8. A heat-sensitive copy-sheet useful in the thermographic reproductionof dilferentially radiation-absorptive graphic originals, comprising apaper-like carrier web coated with a visibly heat-sensitive layerincluding nickel benzoate, a long chain polymeric N,N'-disubstitutedcondensation product of rubeanic acid and hexamethylene diamine, and aminor proportion of a Water-insoluble polymeric film-forming binder.

9. A heat-sensitive copy-sheet useful in the thermographic reproductionof differentially radiation-absorptive graphic originals, including avisibly heat-sensitive layer comprising a salt of a heavy metal and along chain polymeric N,N-disubstituted rubeanic acid condensationderivative of an aliphatic primary polyamine.

10. A heat-sensitive copy-sheet useful in the thermographic reproductionof dillerentially radiation-absorptive graphic originals, including avisibly heat-sensitive layer comprising a salt of a heavy metal and along chain polymeric N,N'-disubstituted polyester condensationderivative of an N,N'-bishydroxyalkyl dithiooxamide and a dianhydride ofa tetracarboxylic acid.

11. A heat-sensitive copy-sheet useful in the thermographic reproductionof differentially radiation-absorptive graphic originals, including avisibly heat-sensitive layer comprising a salt of a heavy metal and along chain polymeric N,N'-disubstituted polyester condensationderivative of an N,N-bishydroxyalkyl dithiooxamide and a polycarboxylicacid.

12. A heat-sensitive copy-sheet useful in the thermographic reproductionof differentially radiation-absorptive graphic originals, including avisibly heat-sensitive layer comprising a salt of a heavy metal and along chain polymeric N,N-disubstituted condensation derivative of anN,N-bishydroxyalkyl dithiooxamide and a diisocyanate.

13. A heat-sensitive copy-sheet useful in the thermographic reproductionof difierentially radiation-absorptive graphic originals, including avisibly heat-sensitive layer comprising a salt of a heavy metal and along chain polymeric N,N'-disubstituted polyester condensationderivative of an N,N-biscarboxyalkyl dithiooxamide and apolyhydroxyalcohol.

References Cited in the file of this patent UNITED STATES PATENTS2,856,387 Jacobson et a1 Oct. 14, 1958 2,910,377 Owen Oct. 27, 19592,916,395 Owen Dec. 8, 1959 2,929,736 Miller et al Mar. 22, 19602,940,956 Smith June 14, 1960 2,995,468 Steiger Aug. 8, 1961 2,999,035Sahler Sept. 5, 1961

1. A HEAT-SENSITIVE COPY-SHEERT USEFUL IN THE THERMOGRAPHIC REPRODUCTIONOF DIFFERENTIALLY RADIATION-ABSORPTIVE GRAPHIC ORIGINALS, INCLUDING AVISIBLY HEAT-SENSITIVE LAYER COMPRISING A SALT OF A HEAVY METAL AND ALONG CHAIN POLYMERIC N,N''-DISUBSTITUTED REBEANIC ACID DERIVATIVE.